This application is based upon Japanese Patent Application Nos. Hei. 10-251399 filed on Sep. 4, 1998, and Hei. 11-250793 filed on Sep. 3, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to flow sensors, and particular to a flow sensor for detecting flow velocity of fluid, and a manufacturing the same.
2. Related Art
Conventionally, flow sensors for detecting flow velocity of fluid flowed along a heater and a temperature detector have been proposed. This kind of flow sensor is generally provided with the heater and the temperature detector, each of which is made of film structure, on a semiconductor substrate (JP-B-6-43906, JP-A-7-174600, or JP-A-9-243423).
FIG. 11 shows a perspective view of a flow sensor according to a related art. A hollow portion 7 is provided in a substrate 1; and a thin film structure portion 2 is provided above the hollow portion 7 so that the thin film structure portion 2 is bridged over the hollow portion 7. In the thin film structure portion 2, a heater 3 is formed in a center portion, and an upper stream temperature detector 5 and a lower stream temperature detector 6 are formed at both peripheral sides. A fluid thermometer 4 for detecting temperature of the fluid is formed on the substrate 1 at a portion, where is different portion of the thin film structure portion 2 and where is an upper stream side of the fluid, which is shown by arrow in FIG. 11.
In this kind of sensor, the heater is operated so that the heater has a certain temperature higher than a fluid temperature, which is detected by the fluid thermometer 4, by a predetermined temperature. The temperature of the upper stream temperature detector 5 falls as the result of losing heat due to flowing of the fluid flows along the upper stream temperature detector 5; whereas the temperature of the lower stream temperature detector 6 rises due to heat generated by the heater 3. The flow velocity of the fluid is measured (detected) based on a temperature difference between the upper stream temperature detector 5 and the lower stream temperature detector 6.
According to the flow sensor having the above-described structure, each of the temperatures of the upper stream temperature detector 5 and the lower stream temperature detector 6 changes with respect to the flow velocity as shown in FIG. 12A; and the temperature difference between the upper stream temperature detector 5 and the lower stream temperature detector 6 changes with respect to the flow velocity as shown in FIG. 12B. As can be understood from FIG. 12B, a characteristic has good linearity at low flow velocity area; however, the linearity becomes worse at high flow velocity area. The reason is as follows. Since the thin film structure portion 2 has an extremely small thermal capacity, the upper stream temperature detector 5 is cooled to around the temperature of the fluid as the flow velocity increases so that a changing ratio of the temperature with respect to the flow velocity is lowered, and a cooling of the lower stream temperature detector by the fluid exceeds a heating by the heater 3. As a result, the temperature difference decreases as the flow velocity increases.
In order to prevent a decreasing of sensibility at the high flow velocity area, some countermeasures are proposed in JP-B-6-68451 or JP-B-4-74672. According to a countermeasure in JP-B-6-68451, the thin film structure portion is made of a metallic film having extremely high thermal capacity to enable to detect at the high flow velocity area. On the contrary, according to a countermeasure in JP-B-4-74672, the temperature detection portion is laminated on the heater with an insulating film interposed therebetween to enable to detect at the high flow velocity area.
However, according to these countermeasures, a structure may be complicated, because they apply the laminated structure of the heater, the temperature detector and the metallic film having extremely high thermal capacity, or apply the laminated structure of the heater and the temperature detector. Furthermore, a method of manufacturing this kind of structure body may be also complicated. Furthermore, in the case where such the laminated structure is applied, particularly where the heater and the temperature detector are made of metallic films and two layers metallic films are formed in the structure body, it is difficult to control a warpage of the structure body due to a stress distribution. Furthermore, thermal stress is generated in the structure body, because the warpage changes due to difference of a thermal expansion coefficient between each material as temperature of the structure body changes. Therefore, when a cooling/heating cycle is repeatedly applied to the structure body as a result of turning on/off of a power supply, or of intermittently turning on, the reliability of the structure body may decrease.
This invention has been conceived in view of the background thus far described and its object is to enable to detect flow velocity in a wide range including high flow velocity area with simple structure.
According to the present invention, a thermal couple film is formed on the substrate at a portion, where is between the heater and the temperature detector, to enhance thermal coupling therebetween. Accordingly, it can prevent the temperature of the temperature detector from falling to around the temperature of the fluid, and therefore, it can detect flow velocity in a wide range including high flow velocity area.
According to another aspect of the invention, a temperature detector has a predetermined pattern so that at least one-portion of the temperature detector is enclosed in an inside of the pattern of the heater. Accordingly, it can prevent the temperature of the temperature detector from falling to around the temperature of the fluid, and therefore, it can detect flow velocity in a wide range including high flow velocity area.